Officials at the Tokyo Electric Power Company (TEPCO), which acts as the operator of the crippled nuclear facility, say there is reason to believe some of the 12 tons of radioactive water has flowed into the Pacific Ocean.
A leak was found in a pipe attached to a temporary decontamination system. The water, once it has been used to cool the reactors, contains massive amounts of radioactive substances and is put into the water-processing facility so it can be recycled for use as a coolant.
"Our officials confirmed that cooling water leaked at a joint in the pipes," a TEPCO spokesman told reporters, adding that "it is possible that some of the water may have flowed outside the facility and poured into the ocean."
This accident is the latest of several leaks of radioactive water at the plant, undermining the government's claim that the shuttered reactors were now under control.
Just last month, about 120 tons of radioactive water leaked at the plant's water decontamination system and about 80 liters (21 gallons) seeped into the ocean, according to TEPCO.
The plant, which is just north-east of Tokyo, was crippled by meltdowns and explosions caused by Japan's massive earthquake and tsunami in March last year.